This invention relates to a multibeam modulation and deflection apparatus for simultaneously driving a single acousto-optic deflector with a plurality of high frequency driving signals to split a beam of laser light incident upon the acousto-optic deflector into a plurality of output beams and also to modulate and deflect the output beams independently of one another. Particularly, the present invention concerns improvements in means for generating such high frequency driving signals to reduce spurious output beams.
It is known that a single acousto-optic deflector can be used to generate a plurality of diffraction-deflected beams of light. For example, such deflectors have begun to be utilized with high speed laser printers which include a laser as the source of light.
Conventional laser printers have comprised an acousto-optic deflector for receiving a beam of laser light, a driving source for simultaneously driving the acousto-optic deflector with a plurality of high frequency driving signals to cause the deflector to produce a plurality of output beams of laser light, each beam independently modulated and deflected with its respective high frequency driving signal, a rotary polygon-mirror for reflecting the output beams from the acousto-optic deflector in a direction perpendicular to the deflection angle of the output beams to form a set of parallel scanning lines, and a printing section for fixing images of the scanning lines formed by the output beams of laser light focussed thereon. In the system, the plurality of high frequency driving signals are individually placed in their ON or OFF state by associated printing data signals individually whereby data represented by the ON or OFF state of the output beams of laser light from the acousto-optic deflector are simultaneously printed, as a plurality of dots, by scanning lines whose number is equal to the number of driving signals. Thus a plurality of lines of data-dots are printed in a parallel relationship along with the scanning by the rotary polygon-mirror.
In the system, it is preferrable to increase the number of parallel printing dots or the number of oscillators generating the high frequency driving signals to obtain a higher printing speed. However, the number of oscillators is limited by the spurious diffraction caused by the nonlinear response of the acousto-optic deflector and an associated driving amplifier for the reasons as will be apparent later.
It is difficult to form an osscilator group including a plurality of oscillators having oscillation phases ideally random and independent of one another, because, with a multiplicity of independent oscillators provided, oscillation phases thereof tend to be pulled in one another. As a plurality of high frequency driving signals having equal phases are applied simultaneously to the acousto-optic deflector, a resultant driving signal thereof may have very large peak-to-peak amplitudes; the amplitudes may be large enough to cause the acousto-optic deflector to respond nonlinearly to the respective driving signals so as to cause the intermodulation of the signal components to thereby effect, in addition to the normal diffraction, spurious diffractions. This is true in the case of the driving amplifier. Therefore, the quality of printed data is much deteriorated. The more the number of the oscillators, the more the adverse influences become.
Accordingly, it is an object of the present invention to provide a new and improved multibeam optical modulation and deflection apparatus for modulating and deflecting a beam of laser light at a super high speed with a high quality by preventing a resultant driving signal composed of a plurality of high frequency driving signals generated by respective oscillators from having high peak-to-peak amplitudes so as to thereby avoid the nonlinear response of both an acousto-optic deflector and an associated amplifier so as to thereby reduce the spurious output beams.